This invention relates to an attenuator having a high input impedance and several switched levels and intended to attenuate a high-frequency electric signal introduced at an input terminal of the attenuator, the latter supplying at a low impedance output terminal an output signal corresponding to the attenuated input signal.
The invention further relates to an oscilloscope and an active probe for an oscilloscope comprising such an attenuator.
An attenuator of this kind is known from U.S. Pat. No. 4,525,688, which discloses an input circuit for an oscilloscope comprising an attenuator having a high input impedance and operating in a wide frequency band. Instruments, such as oscilloscopes, which have to carry out quantitative measurements of high accuracy, require the use of attenuation input circuits having a high input impedance so that the load impedance formed by attenuator and oscilloscope do not influence the source to be measured.
For this purpose, the aforementioned patent describes a controllable attenuator constituted by a cascade arrangement of dividers comprising resistors and capacitors and each constituted by a first parallel circuit R.sub.a C.sub.a having fixed elements, by a second parallel circuit R.sub.b C.sub.b whose capacitance C.sub.b is adjustable, by two adjustable capacitances C.sub.c, C.sub.d and by two interruptors S.sub.a and S.sub.b. The three adjustable capacitances C.sub.b, C.sub.c and C.sub.d serve to control the frequency response and level responses of the attenuator. The two interruptors S.sub.a and S.sub.b are controlled so as to utilize or not to utilize the cell.
However, this attenuator has several disadvantages. The adjustments of the different capacitances which have to be effected for each level are numerous, can be carried out only with difficulty and are not very stable taking into account the accuracy of more than 10.sup.-2 required by the envisaged applications. These adjustments require expensive components. The interruptors S.sub.a and S.sub.b are arranged in the high-impedance part of the attenuator so that their high parasitic capacitances cause them to be poorly adapted to this application. Their dimensions in fact cannot be reduced sufficiently and this can lead to input capacitances of the attenuator that can amount to 10 to 20 pF.